1. Field of the Invention:
The present invention relates to an improved wet oxidation process using ozone for the destruction of noxious gases obtained from sewage sludge drying operations.
2. Description of the Prior Art:
The use of ozone for the control of odors generated by various industrial operations is well known. For example, ozone has been utilized for odor control in distilleries, breweries, paper mills, chemical manufacturing facilities, wastewater treatment facilities, rendering plants, fish processing plants, and the like.
One method of treatment is the so-called "direct injection" method wherein ozone is injected directly into the odor-laden gas and contacted therewith using a suitable gas-gas contactor. The amount of ozone required and contact time varies depending primarily on the nature and concentration of the malodorous contaminants. While the direct injection technique is useful for the deodorization of readily oxidizable contaminants present in relatively low concentrations, it is not totally effective where high concentrations of odorants and/or organic odorants which are difficultly oxidizable are present. The "wet oxidation" process has proven to be much more effective and desirable for the treatment of gases of the latter type.
Wet oxidation processes involve contacting the odor-laden gases with ozone in the presence of water. Typically this is accomplished by contacting the waste gas with recirculating water which is saturated or essentially saturated with ozone. Typical wet oxidation procedures are described by D. L. B. Arnold, Chem. Ind. (London), 1974(22), 899-902 and R. Anderson and G. F. Greaves, Water Pollution Control (Maidstone, England), 82(1), 18-20(1983). Because of the higher dosage of ozone possible and the more efficient contact obtained with the wet oxidation (gas-liquid) process, it is considerably more effective than gas-gas contact methods for the treatment of noxious gases containing relatively high concentrations of malodorous organic materials. The wet oxidation method has therefore been widely accepted for use for the chemical oxidation of odors emanating from sewage sludge treatment operations since such gases typically have high concentrations of highly objectionable putrid organic nitrogen- and sulfur-containing substances, particularly skatole, methyl disulfide, dimethyl sulfide, methyl mercaptan, putrescine and cadavarine.
There are, however, two problems associated with the use of the wet oxidation process for the treatment of gases obtained from sludge treatment operations. The first is the inability to consistently consume all of the ozone present in the system. An appreciable excess of ozone is typically utilized in the process to compensate for variations in the odorous gas and to obtain optimum oxidation. This often results in ozone "breakthrough," i.e., the presence of unreacted ozone in the deodorized gas. Since there are restrictions on the amount of residual ozone which can be vented to the atmosphere, scrubbers or some other ozone destruct means must therefore be provided. It would be highly advantageous if ozone breakthrough could be eliminated and the use of costly scrubbing/ozone destruct equipment avoided.
The other problem is the inability to completely eliminate secondary odors present in the exhaust gas. While the highly objectionable putrid odor is readily removed and undetectable after treatment, the treated exhaust gas still does not have the quality of fresh air. A persistent secondary odor, generally characterized and referred to herein as a "sweet sulfite" odor and believed to result from inorganic sulfite and alkyl sulfide partial oxidation products, is evident in the exhaust gas obtained from the prior art wet oxidation processes. It would be highly desirable if a process were available wherein the sweet sulfite odor was eliminated from the treated exhaust gas.